1. Field of the Invention
The invention provides a process for preparing low-chlorine content ethylene copolymers by solution polymerization using vanadium-containing mixed catalysts and in particular low-chlorine activators.
2. Description of the Background
It is known that ethylene can be polymerized together with other .alpha.-olefins and, if desired, non-conjugated dienes in the presence of organometallic (Ziegler-Natta) mixed catalysts. The polymerization is carried out in organic solvents. The catalysts used are transition metal compounds of the subgroups IV to VI of the Periodic Table (usually vanadium compounds in the oxidation states 3+ to 5+) together with organometallic compounds of the main groups I to III (usually organoaluminum compounds). Such catalyst systems have a very high initial activity which, however, rapidly decreases as a result of the rapid reduction of the vanadium to low, polymerization-inactive oxidation states (e.g. 2+). To achieve industrially acceptable polymer yields, use is made of activators which reoxidize the vanadium to polymerization-active oxidation states.
The most efficient activators for the vanadium-containing catalysts are chlorine-containing substances. In practice, polychlorinated compounds such as, for example, trichloroacetic esters (DE-A 15 70 726, Hercules Powder Co.), perchlorocrotonic esters (DE-B 15 95 442, Hu/ ls AG) or hexachlorocyclopentadiene (DE-A 14 95 698, Farbwerke Hoechst AG) have proven useful. However, these activators have the decisive disadvantage that the resulting copolymers have a chlorine content which is too high. Some polymer properties, primarily the ageing resistance, are impaired by the chlorine content. Furthermore, chlorine-containing polymers lead to increased corrosion during processing of the chlorine containing polymer. Lower chlorine content compounds, e.g. monochloromalonic and dichloromalonic esters (CA 1 272 847, Uniroyal Chemical Co.) usually have a low activity. In practice this use of such compounds leads to a low solids content in the polymer solution. To overcome this deficiency, disproportionately high ratios of activator/VOCl.sub.3 would be needed, which is very disadvantageous economically.
In recent years, effective compounds having lower chlorine contents have been claimed as activators, e.g. dichlorophenylacetic esters (EP-B-0 044 119 and 0 044 595, Stamicarbon B. V.). However, even when such activators are employed, the chlorine concentration in the polymer can be reduced to the low level required only by expensive polymer washing. A need, therefore, continues to exist for a catalyst activator which leads to lower chlorine content in the product polymer.